Conventional hydrocarbon conversion processes can be utilized for producing relatively high-value hydrocarbons, e.g., light olefins such as ethylene, propylene, butylene, etc., from relatively low-value hydrocarbon-containing feeds, such as methane, ethane, propane, naphtha, gas oil, crude oil, heavy oil, etc. One such conventional process is steam cracking, a form of thermal pyrolysis where a feed comprising substantially-saturated hydrocarbon is combined with steam, the steam-hydrocarbon mixture then being pyrolysed in one or more radiantly-heated pyrolysis tubes. Steam cracking is an endothermic process, with heat being provided by combusting fuel in one or more furnaces. The pyrolysis tubes are generally located in a furnace box, and a conventional olefins-production facility may include a plurality of furnaces. The radiant heat flux to the pyrolysis tubes can be derived, e.g., from a plurality of burners located in or near the furnace boxes.
There have been significant efforts directed toward increasing the overall efficiency of pyrolysis processes such as steam cracking. One method for increasing efficiency, as disclosed in U.S. Pat. No. 4,287,377, involves burning a mixture of fuel and pre-heated air in the burner, the air being pre-heated in successive compression, heating, and expansion stages of a gas turbine. Fuel and the expanded combustion effluent are combusted in a burner that is located in the cracking furnace, in order to heat pyrolysis tubes. The process exhibits an efficiency gain because heating the air in a gas turbine enables the use of heavier (less expensive) fuels in the burner. An additional efficiency gain is obtained by utilizing work from the gas turbine's expansion zone for compressing the pyrolysis product in the olefin recovery train. Although utilizing a higher air temperature in the burner's air supply is generally beneficial, the ultimate air temperature is limited by the amount of expansion in the gas turbine's expansion zone. Less expansion generally provides burner air of a higher temperature, but also lessens the expansion zone's ability to produce work.
The gas turbine itself can be made more efficient, e.g., by pretreating the gas turbine's fuel, as disclosed in U.S. Pat. No. 5,669,216. The patent discloses decompressing combustion effluent from the gas turbine's combustion chamber in the turbine's expander. Heat is indirectly transferred from the expanded combustion effluent to a hydrocarbon feed in order to upgrade the feed to a higher heat-value fuel for the gas turbine's combustion chamber. As an example, the patent discloses using the indirect heating to upgrade a biogas, LPG, naphtha, or kerosene feed by steam cracking. Further efficiencies can be realized by indirectly transferring heat from the expanded combustion effluent to produce steam for powering a steam turbine. Undesirably, utilizing the steam-cracked product as a gas turbine fuel represents the loss of valuable olefinic products.
It is desired to utilize gas turbine technology to further increase hydrocarbon conversion process efficiency, and in particular to obtain an increase in both efficiency and the amount of recovered olefinic products. It is also desirable to do so without appreciably lessening the amount of power produced by the process, e.g., to obtain an increase in both efficiency and the amount of recovered olefinic products without appreciably decreasing the net work obtained from the gas turbine.